The present invention relates to using analog devices with digital switching systems (DSS), for example Digital private branch exchanges (PBXs) and digital Key System Units (KSUs), which provide connections to digital devices.
A digital switching system is typically used by an organization for facilitating both internal communications within the organization and also communication with the outside world. These devices are often called Customer Premise Equipment (CPE), as the organization is typically a customer of the public telephone company. A DSS switches internal connections, such that a plurality of digital sets each connected to the DSS can communicate with each other, and also with the outside world via outside connections. A typical DSS will have outside connections which comprise analog connections to a central office (CO) switch for communication with the outside world via the Public Switched Telephone Network (PSTN), and connections to local digital telephone sets. Such a DSS allows a plurality of digital devices to share one or more analog lines. The DSS will convert between analog signals carried by the analog lines and digital signals used by the digital sets.
A DSS will typically only support digital devices. There are known advantages gained by using digital sets. However, one disadvantage of such a DSS is that analog devices, for example facsimile (fax) devices, modems, and analog telephones are not compatible with digital switching systems.
In the past, one way of using such an analog device was to have a dedicated analog line to the CO for use by the analog device. However, this has the disadvantage of being expensive to the customer, who has to pay for a dedicated line that will be underutilized, as the device is typically not used on a constant basis. Furthermore, as the line is dedicated, it can not be shared with other devices when the analog device is not using the line.
One prior art solution was to use an Analog Terminal Adapter (ATA). This is a device which is compatible with the DSS, and is considered by the DSS to be a digital device connected to the DSS. The ATA includes an analog line interface circuit which includes Digital to Analog converters for converting between digital and analog signals, as well as providing supervision (e.g., ringing voltage, off hook detection, etc.) to the analog device. Such a device is expensive and also has the disadvantage of not providing a true analog interface to the CO. Signal degradation can occur due to the combination of A/D conversion in the DSS and then D/A conversion in the ATA (and vice a versa). A user of an analog POTS (Plain Ordinary Telephone Service) telephone is unlikely to notice any effects arising from this double conversion. However, other analog devices, such as high speed modems (e.g., 56 Kilobit/second modems) will suffer signal degradation, which is unacceptable for data transmission and results in a decrease in data transmission rate. Furthermore, such a solution will not function in the event of a power failure (or at least will not function without battery backup).
Other prior art solutions include emergency transfer relay for connecting a POTS telephone to an analog line in the event of a power failure. Such a solution provides fall back to POTS in a relatively cheap manner, but does not allow for the use of a fax or modem. Furthermore, a call made during a power failure will be automatically disconnected when the DSS powers up and grabs the line.
Therefore, there exists a need for a system which will allow an analog device to share an analog line with a DSS which overcomes these problems in a cost effective manner.
An object of the present invention is to provide an auxiliary connection mechanism for allowing a Digital Switching System (DSS) to share an analog line with an analog device (via an auxiliary device port). Both the analog device and the DSS should be able to receive incoming calls on the analog line. However, once one of the devices (either the analog device or the DSS) uses the line, the other device should not access the line.
Thus there is provided an auxiliary connection mechanism for releasably connecting both the auxiliary device port and the DSS to the analog line. Both the auxiliary device port and the DSS are connected to the analog line when the line is idle, so both the auxiliary device port and the DSS can receive incoming calls. A busy line detection circuit is included to detect whether the line is being used. A controller monitors the status of the analog line and controls access to the analog line, in order to prevent the auxiliary device and the DSS from using the analog line at the same time.
Such an auxiliary connection mechanism is preferably integrated with the DSS. However, in one embodiment, the auxiliary connection mechanism comprises a separate box interposed between the analog line and the DSS.
In accordance with a broad aspect of the present invention there is provided a digital switching system comprising:
at least one analog interface for connecting to at least one analog line;
a digital interface for connecting to a plurality of digital lines;
a switching mechanism connected between said at least one analog interface and said digital interface for making connections between said at least one analog line and said digital lines;
an auxiliary device port;
an auxiliary connection mechanism for connecting said auxiliary device port to said at least one analog line including: a first connection for connecting said analog line to said analog interface, and a second connection for connecting said analog line to said auxiliary device port.
a controller for controlling the states of said connections.
According to a further aspect of the invention which provides a supervised emergency transfer feature, the controller checks the status of the busy line circuit upon power up prior to providing the switching mechanism with access to the analog line, and does not provide said access while said analog line is in use by a device connected to said auxiliary device port.
According to another aspect of the invention there is provided an auxiliary connection mechanism for connecting an analog line to either a digital switching system (DSS) or an analog device port, said auxiliary connection mechanism comprising:
a first switch for connecting said analog device port to said analog line;
a second switch for connecting said digital switching system to said analog line;
a controller for controlling said switches so that both the digital switching system and said analog device port are connected to said analog line when said analog line is idle, and for disconnecting one of said switches when the other device uses the line.